The present invention relates to a technical field of an intake device of an engine.
Conventionally, in order to increase a tumble flow of intake air flowed into a combustion chamber of an engine from an intake port of a cylinder head of the engine, efforts have been made to improve a shape of a throat portion of the intake port. For example, JP2010-174702A discloses a shape of a throat portion of an intake port in which an edge is formed in a first wall surface (upper wall surface) of the throat portion, and a convexly curved surface is formed closer to a side of a guide hole of a valve stem guide (upstream side) than the edge in the first wall surface. Further, a part of a second wall surface (lower wall surface) of the throat portion near a downstream end thereof is extended toward an extended line of a center axis of the guide hole (a center axis of an intake valve) to form an edge.
Normally, two intake ports are formed for every cylinder. Therefore, it can be considered to form throat portions of the two intake ports to respectively have a shape which would increase a tumble flow of intake air (e.g., the shape in JP2010-174702A) so that a strength of a tumble flow of intake air formed within the combustion chamber when a flow of the intake air into the combustion chamber is assumed to be caused only from one of the two intake ports becomes substantially the same as a strength of a tumble flow formed within the combustion chamber when the flow of the intake air into the combustion chamber is assumed to be caused only from the other intake port.
However, in this case, when the intake air flows into the combustion chamber from both of the intake ports, a mixing performance of the intake air flowed into the combustion chamber from one of the intake ports with the intake air flowed into the same from the other intake port degrades, and, in particular, if the intake air is forcibly induced by a forced induction system, the mixing performance degrades even more. Therefore, especially within a forcibly inducing range of the engine where the intake air is forcibly induced by the forced induction system, the mixing performance of fuel with the intake air degrades and, thus, combustibility of a mixture gas of the fuel and the intake air degrades and it becomes difficult to improve fuel consumption.